linux-kernel - [PATCH 110/110] lib/vdso: Improve do_hres() and update vdso data unconditionally

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Date:   Mon, 21 Oct 2019 20:52:00 +0800
From:   Huacai Chen <chenhc@...ote.com>
To:     Andy Lutomirski <luto@...nel.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Vincenzo Frascino <vincenzo.frascino@....com>,
        chenhuacai@...il.com, linux-kernel@...r.kernel.org,
        Huacai Chen <chenhc@...ote.com>, stable@...r.kernel.org,
        Arnd Bergmann <arnd@...db.de>,
        Paul Burton <paul.burton@...s.com>, linux-mips@...r.kernel.org,
        linux-arm-kernel@...ts.infradead.org
Subject: [PATCH 110/110] lib/vdso: Improve do_hres() and update vdso data unconditionally

In do_hres(), we currently use whether the return value of __arch_get_
hw_counter() is negative to indicate fallback, but this is not a good
idea because:

1, ARM64 returns ULL_MAX but MIPS returns 0 when clock_mode is invalid;
2, For a 64bit counter, a "negative" value of counter is actually valid.

It is sure that MIPS has a bug when clock_mode is invalid and should
return ULL_MAX as ARM64 does (Vincenzo has already submitted a patch).
But do_hres() can still be improved so we use U64_MAX as the only
"invalid" return value -- this is still not fully correct, but it is
the simplest fix and has no problem in most cases (we can hardly see a
64bit counter overflow).

By the way, currently update_vdso_data() and update_vsyscall_tz() rely
on __arch_use_vsyscall(), which causes __cvdso_clock_getres() and some
other functions get wrong results when clock_mode is invalid. So, we
update vdso data unconditionally.

Fixes: 00b26474c2f1613d7ab894c5 ("lib/vdso: Provide generic VDSO implementation")
Fixes: 44f57d788e7deecb50484353 ("timekeeping: Provide a generic update_vsyscall() implementation")
Cc: stable@...r.kernel.org
Cc: Arnd Bergmann <arnd@...db.de>
Cc: Paul Burton <paul.burton@...s.com>
Cc: linux-mips@...r.kernel.org
Cc: linux-arm-kernel@...ts.infradead.org
Signed-off-by: Huacai Chen <chenhc@...ote.com>
---
 kernel/time/vsyscall.c  | 9 +++------
 lib/vdso/gettimeofday.c | 2 +-
 2 files changed, 4 insertions(+), 7 deletions(-)

diff --git a/kernel/time/vsyscall.c b/kernel/time/vsyscall.c
index 4bc37ac..5ee0f77 100644
--- a/kernel/time/vsyscall.c
+++ b/kernel/time/vsyscall.c
@@ -110,8 +110,7 @@ void update_vsyscall(struct timekeeper *tk)
 	nsec		= nsec + tk->wall_to_monotonic.tv_nsec;
 	vdso_ts->sec	+= __iter_div_u64_rem(nsec, NSEC_PER_SEC, &vdso_ts->nsec);
 
-	if (__arch_use_vsyscall(vdata))
-		update_vdso_data(vdata, tk);
+	update_vdso_data(vdata, tk);
 
 	__arch_update_vsyscall(vdata, tk);
 
@@ -124,10 +123,8 @@ void update_vsyscall_tz(void)
 {
 	struct vdso_data *vdata = __arch_get_k_vdso_data();
 
-	if (__arch_use_vsyscall(vdata)) {
-		vdata[CS_HRES_COARSE].tz_minuteswest = sys_tz.tz_minuteswest;
-		vdata[CS_HRES_COARSE].tz_dsttime = sys_tz.tz_dsttime;
-	}
+	vdata[CS_HRES_COARSE].tz_minuteswest = sys_tz.tz_minuteswest;
+	vdata[CS_HRES_COARSE].tz_dsttime = sys_tz.tz_dsttime;
 
 	__arch_sync_vdso_data(vdata);
 }
diff --git a/lib/vdso/gettimeofday.c b/lib/vdso/gettimeofday.c
index e630e7f..5a31643 100644
--- a/lib/vdso/gettimeofday.c
+++ b/lib/vdso/gettimeofday.c
@@ -50,7 +50,7 @@ static int do_hres(const struct vdso_data *vd, clockid_t clk,
 		cycles = __arch_get_hw_counter(vd->clock_mode);
 		ns = vdso_ts->nsec;
 		last = vd->cycle_last;
-		if (unlikely((s64)cycles < 0))
+		if (unlikely(cycles == U64_MAX))
 			return -1;
 
 		ns += vdso_calc_delta(cycles, last, vd->mask, vd->mult);
-- 
2.7.0